Various problems are generally presented in treatment involving surgical operation to a vital body. For example, a patient undergoing an operation must withstand a long-time operation, has a relatively high danger of infection and often causes invasion. A surgeon, as well, is forced to endure intense concentration for the long period of operation.
In order to permit lightening such various stress and performing a necessary operation more safely and easily, catheters, guide wires and embolizing materials for occluding blood vessels and the like, and other various medical instruments have been developed in recent years and been put to practical use. With the recent advancement of medical instruments such as catheters and guide wires, an intravascular operation in which the intended affected part is approached through a blood vessel comes to be performed and often applied to treatments for diseases such as arteriovenous malformation, cerebral aneurysm and carotid-cavernous fistula.
As a method for treating aneurysm or the like, which causes little invasion, there is currently known vascular embolization in which an intracorporeally indwelling member composed of a metal, which does not exert an adverse influence on a vital body, such as platinum is left within aneurysm. In this method, a medical wire with the intracorporeally indwelling member connected to the leading end part of a delivery wire is inserted into a catheter arranged in the vital body, the wire is operated under observation by a radiography to guide the intracorporeally indwelling member to a desired position within the vital body, whereby the intracorporeally indwelling member is caused to reach the intended site, and the intracorporeally indwelling member is released in this state.
As a method for releasing the intracorporeally indwelling member, there are known mechanical means and electrical means. For example, as an electrically releasing means, there is a means of supplying an electric current between a conductive wire and a counter electrode connected to the vital body from the outside to decompose and fuse a connecting member (see, for example, Patent Art. 1 and Patent Art. 2).
Patent Art. 2 discloses a medical wire for vascular embolization in which an intracorporeally indwelling member is connected to the leading end part of a conductive wire through a thermally-fusible rod-like connecting member composed of polyvinyl alcohol. According to this medical wire, a high-frequency electric current is applied between the conductive wire and the counter electrode, whereby the leading end part of the conductive wire functions as an electrode for heating to thermally fuse the connecting member in a moment, and the intracorporeally indwelling member is separated from the conductive wire, so that the medical wire is said to have merits that the time required for a surgical operation can be shortened and burdens imposed on a patient and a surgeon can be lightened.    Patent Art. 1: Japanese Patent Registration No. 3007022; and    Patent Art. 2: Japanese Patent Registration No. 2880070.
In the medical wire formed by using such a connecting member composed of thermally-fusible polyvinyl alcohol as disclosed in Patent Art. 2, however, the high-frequency current flows through an electrolyte in the vital body when the current is applied between the conductive wire and the counter electrode connected to the vital body. At this time, the leading end part of the conductive wire functions as a heating electrode to fuse the connecting member.
When it is specifically described, this conventional medical wire is formed by a conductive wire body 50, a connecting member 52 and an intracorporeally indwelling member 54 as illustrated in FIG. 9. More specifically, the leading end part of the conductive wire body 50 is formed by a coil piece 56, and the coiled intracorporeally indwelling member 54 is connected to this coil piece 56 through the thermally-fusible connecting member 52. An insulating coating film 58 is formed on an external peripheral surface of a region of the wire body 50 other than the leading end part and the base end part thereof, and a heating electrode portion is formed by an exposed coil portion E of the coil piece 56, which is exposed without the coating film 58.
A high-frequency current is supplied from the base end part 60 of the wire body 50, whereby Joule heat is generated at the heating electrode portion by an electric resistance produced between the electrode portion and an electrolyte solution of a body fluid of a vital body contacting therewith, and a temperature around the heating electrode portion is raised by this Joule heat to fuse the thermally-fusible connecting member 52, thereby separating the intracorporeally indwelling member 54 from the wire body 50.
In order to fuse the connecting member in such a manner, it is necessary to raise the temperature of the heating electrode portion. In order to achieve this, it is necessary to convert electric energy by electric power from a high-frequency power source device to thermal energy (Joule heat) by a resistance in the electrolyte or the like around the heating electrode portion. The quantity of this Joule heat generated is determined by the degree of an electric resistance value around the heating electrode portion.
In an actual operation for leaving the intracorporeally indwelling member, the resistance value in the heating electrode portion is greatly affected by an ambient environment in which the heating electrode portion is located. For example, in the treatment for aneurysm, it is generally necessary to fill a plurality of intracorporeally indwelling members each composed of a metal such as platinum at a considerably high density into the aneurysm. Therefore, when an intracorporeally indwelling member is left in the latter half of the treatment, the circumference of the heating electrode portion of the wire body is in a state that a plurality of the intracorporeally indwelling members previously arranged has been present.
When such an intracorporeally indwelling member previously arranged is in a state that it has unexpectedly come into contact with the exposed heating electrode portion of the wire body, a part of the high-frequency current supplied to the heating electrode portion flows to the intracorporeally indwelling member contacting to cause leak, and after all, a state that the surface area of a member acting as an electrode has been increased is created. As a result, the resistance value in the electrode portion becomes lowered.
In the state that the resistance value has been lowered as described above, the quantity of Joule heat generated becomes decreased. As a result, the thermally-fusible connecting member is not completely fused, and thus the intracorporeally indwelling member remains unseparated.
More specifically, when the area of the electrode increases in the case where the electric power supplied is fixed, a heating area increases though the total quantity of Joule heat generated is the same, so that the rising breadth of the temperature decreases. Thus, the connecting member cannot reach a fusing temperature, and so the fusing of the connecting member cannot be completely performed. After all, the intracorporeally indwelling member cannot be released unless a current for fusion is supplied repeatedly. In such a case, a doctor who is a surgeon is required to conduct a confirming operation that the wire is pulled back while viewing an image through an X-ray equipment for radiography in order to confirm the fact that the intracorporeally indwelling member has been separated from the wire. When the intracorporeally indwelling member is not separated from the wire as a result of this confirming operation, however, it is necessary to store the intracorporeally indwelling member in the aneurysm by operating a delivery wire again, and moreover seeking an appropriate position where the intracorporeally indwelling member previously arranged comes into no contact with the electrode portion of the wire body. In the worst case, there is a possibility that the other intracorporeally indwelling member previously arranged in the same aneurysm may be moved or forced out of the aneurysm when the wire has been pulled back, so that burdens and risks imposed on the patient and the doctor become great.